Process for preparing the through hole walls of a printed wiring board for electroplating

ABSTRACT

In the process for electroplating the walls of through holes in a laminated printed wiring board comprised of at least one non-conducting layer laminated to at least two separate conductive metal layers, which comprises the steps: 
     (a) contacting said printed wiring board having said through holes with a liquid dispersion of carbon blaock comprised of: 
     (1) carbon black particles having an average particle size of less than about 3.0 microns in said dispersion; 
     (2) an effective dispersing amount of a surfactant which is compatible with said carbon black; and 
     (3) a liquid dispersing medium, wherein the amount of carbon black is sufficient to coat substantially all of said non-conducting surfaces and is less than about 4% by weight of said liquid dispersion; 
     (b) separating substantially all of the liquid dispersing medium from said applied dispersion, thereby depositing said carbon black particles in a substantially continuous layer on said non-conducting portions of said hole walls; 
     (c) microetching said metal layers of said printed wiring board to remove deposited carbon black therefrom; 
     (e) electroplating a substantially continuous metal layer over the deposited carbon black layer on said non-conducting portions of hole walls, thereby electrically connecting said metal layers of said printed wiring board; 
     wherein said improvement comprises: 
     contacting said deposited carbon black particles after step (b) and before step (c) with aqueous alkaline solution of an alkali metal borate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved process for preparing the throughhole walls of a printed wiring board (PWB) for electroplating.

2. Description of Related Art

For the past quarter century the printed wiring board industry hasrelied on the electroless copper deposition process to prepare throughhole walls in printed wiring boards for electroplating. These platedthrough hole walls are necessary to achieve connections between twometal circuit patterns on each side of a printed wiring board or, inaddition to this, between the inner layer circuit patterns of amultilayer board.

The electroless deposition of copper onto the through hole wallstypically consists of precleaning a PWB and then processing according tothe following sequence of steps:

Step 1. Preactivator

Step 2. Pd/Sn Activator

Step 3. Rinse

Step 4. Acellerator

Step 5. Rinse

Step 6. Electroless Copper Deposition

Step 7. Electroplating

These processed boards may also be photoimaged before the electroplatingprocess. Typically, the deposited copper layer on each through hole wallis about 1+0.2 mil thick.

Conventional electroless processes have several commercialdisadvantages. They require a relatively long process time. The multipletreatment baths have complex chemistry which may require constantmonitoring and individual ingredients which may require separatereplenishment. The palladium/tin activator also may require expensivewaste treatment. Furthermore, these electroless process baths may bevery sensitive to contamination. Finally, the multiplicity of rinsebaths may require large amounts of water.

Prior to the electroless method of plating through holes, graphite wasemployed to prepare the walls of the through holes for plating. Forexample, U.S. Pat. No. 3,099,608, which issued to Radovsky et al. onJuly 30, 1963, teaches a process for preparing the through hole walls ofprinted circuit boards for electroplating by initially depositing insaid through holes a thin electrically non-conductive film of palladiummetal in at least a semi-colloidal form. The patent discloses thatgraphite had been used previously as a conductive layer forelectroplating thereon. See column 1, lines 63-70 and column 4, line 72to column 5, line 11. These patentees noted several deficiencies withthat graphite process including lack of control of the graphiteapplication, poor deposit of the resultant electroplated metal,non-uniform through hole diameters, and low electrical resistance of thegraphite.

U.S. Pat. No. 3,163,588, which issued to Shortt et al on Dec. 29, 1964,also mentions that graphite or its equivalents may be employed to renderthrough hole walls of electric circuit boards conductive for laterelectroplating metals thereon. See column 3, line 45 to column 4, line2.

U.S. Pat. No. 4,581,301, which issued to Michaelson on Apr. 8, 1986,teaches the application of a seed layer of conductive particles, such as"carbon", on the wall of through holes before electrolytically platingcopper over the seed layer. This reference does not explicitly teach theuse of a continuous layer of surfactant or carbon black in the seedlayer, and does not recognize the advantage of using very smallparticles of carbon black such as presently claimed. See column 7, lines63-66 which refer to particles passing through a 400 mesh screen. A 400mesh screen is equivalent to about 35 microns.

Separately, graphite has been employed in numerous processes forpreparing a non-conducting material for a metal coating or plating. Forexample, U.S. Pat. No. 409,096, which issued to Alois Blank on Aug. 13,1889, teaches a process for applying copper to asbestos roofing materialwhich comprises first applying powdered plumbago (graphite) in avolatile liquid such as varnish to the surface of the asbestos, thenevaporating the volatile liquid to coat the asbestos fibers with fineparticles of plumbago. The plumbago coated asbestos sheets are thenimmersed in a copper electroplating solution and electric current isapplied to the coated asbestos sheet to form a thin film of copperthereon. The copper coated sheet is then immersed in a bath of moltenmetal such as tin, lead, or zinc, and is then removed from the moltenbath to effect solidification of the molten metal. The resulting metalcoated asbestos sheet is described as being relatively flexible, anon-conductor of heat and substantially fireproof.

U.S. Patent No. 1,037,469, which issued to Goldberg on Sept. 3, 1912,and U.S. Pat. No. 1,352,331, which issued to Unno on Sept. 7, 1920,disclose processes for electroplating non-conducting materials by firstcoating the non-conducting material with wax, then coating the wax witha slurry of finely divided particles of graphite or other metal,followed by electroplating of the dust-coated surface with copper orother metal. Neither of these processes are particularly suitable foruse in coating the hole walls of circuit boards because the holes arenormally extremely narrow in diameter and immersing in wax would tend toplug the hole and prevent coating the hole walls with an electroplatingmaterial.

U.S. Pat. No. 2,243,429, which issued to Laux on May 27, 1941, disclosesa process for electroplating a non-conductive surface by "graphiting" athin layer onto the non-conducting surface followed by applying a copperlayer electrolytically and "finally a further electrolytic deposit ofanother metal" is placed thereon.

Separately, caroon black formulations have been employed as conductivecoatings for non-conductive materials. For example, U.S. Pat. No.4,035,265, which issued to Saunders on July 12, 1977, disclosesconductive paint compositions containing both graphite and carbon blackalong with air-hardenable binder. These paints are suitable forapplication to the walls of a building for use as a heating element.

U.S. Pat. No. 4,090,984, which issued to Lin et al on May 23, 1978,teaches a semi-conductive coating for glass fibers comprising (a) apolyacrylate emulsion; (b) electrically conductive carbon blackdispersion and (c) a thixotropic gelling agent. The conductive carbonblack dispersions employed are those comprising electrically conductivecarbon black dispersed in from about 3 to about 4 percent by weight of asuitable dispersing agent.

U.S. Pat. No. 4,239,794, which issued to Allard on Dec. 16, 1980,teaches dispersing a conductive carbon black in a latex binder with aselected dispersing agent, then impregnating this carbon blackdispersion into a non-woven fibrous web followed by drying any residualwater, leaving a thin coating of carbon black dispersed on the surfacesof said fibers.

U. S. Pat. Nos. 4,619,741; 4,684,560 and 4,724,005, which issued to KarlL. Minten and Galina Pismennaya, on Oct. 28, 1986; Aug. 4, 1987; andFeb.9, 1988, respectively, teach a process of electroplating the throughholes of a PWB which is a significant improvement over the knownelectroless techniques. By this process, a liquid dispersion of carbonblack particles is first applied to the non-conductive portions of thethrough holes; then the liquid dispersion medium is separated (i.e.evaporated) from the carbon black particles, thereby depositing asubstantially continuous layer of carbon black particles on thenon-conductive surfaces of the through holes; and next a substantiallycontinuous metal layer is electroplated over the deposited carbon blacklayer. This process of Minten and Pismennaya has several advantages overthe known electroless techniques including the eliminaton of thepreactivator, the Pd/Sn activator and the accelerator; less possibilityof pollution problems; better bath stability; and fewer possible sidereactions. This disclosure of the above-mentioned U.S. Patents of Mintenand Pismennaya is incorporated herein by reference in their entirety.

While this Minten and Pismennaya process in itself teaches an effectivemeans for electroplating through holes, there is still a need to improvethe overall quality (i.e. achieve a void-free copper deposit) for alltypes of printed wiring boards, especially multilayer boards.Furthermore, while a preferred embodiment disclosed in this applicationemploys a microetch to remove carbon black particles from the copper onmetal surfaces of the PWB before electroplating, there is still a needto more thoroughly remove the carbon black off of these metal surfaces.In other words, it desirable to make that microetching step moreeffective.

This is especially necessary for copper foil rims of the though holes ofthe PWB and the inner copper foil layers inside the through holes of thePWB to be completely cleaned before the micro-etch step. If there is arelatively thick carbon black layer remaining on those rims or innercopper foil layers during the electroplating step, that carbon blackmight insulate those rims or copper foil layers from the plated-oncopper layer and, thus prevent a good electrolyic connection betweenthem, and, if the carbon black layer is plated over in these rim andinner layer areas, an adhesion failure may occur. Also, in someinstances, there is loose excess deposits of carbon black on thenon-conductive (e.g. epoxy/fiberglass) portions of the through holes. Ifelectroplating proceeds with these loose excess deposits present, theplating surface over these portions may not be smooth.

Improvement and modifications of this Minten and Pismennaya process areshown in U.S. Pat. Nos. 4,622,107 (Piano); 4,622,108 (Polakovic andPiano) and 4,631,117 (Minten, Battisti and Pismennaya) and 4,718,993(Cupta and Piano). The first of these patents teaches the use of agas-forming compound (e.g. sodium carbonate) to remove loose or easilyremovable carbon black particles in the through holes. The second ofthese patents teaches the contacting of an alkaline hydroxidepre-conditioning solution to the through hole walls before applicationof the carbon black dispersion so that the carbon black dispersion willhave better adhesion to the walls. The third listed patent teaches theuse of this carbon black dispersion as a pre-activator for electrolessplating of the through holes. The fourth teaches the use of a alkalinesilicate solution before the carbon black dispersion. These four U.S.Pats. are incorporated herein by reference in their entireties.

It is a primary object of this invention is to provide improvedelectroplating process for applying a conductive metal layer to thethrough hole walls of printed wiring boards over the process disclosedin the above-noted Minten and Pismennaya patents.

It is another object of this invention to provide an even moreeconomical process for applying a conductive metal layer to the surfacesof non-conducting layers of printed wiring boards than presently knownelectroless processes.

BRIEF DESCRIPTION OF THE INVENTION

Accordingly, the present invention accomplishes the foregoing objects byproviding an improvement to the process for electroplating the walls ofthrough holes in a laminated printed wiring board comprised of at leastone non-conductive layer laminated to at least two separate conductivemetal layers, which comprise the steps:

(a) contacting said printed wiring board having said through holes witha liquid dispersion of carbon black comprised of:

(1) carbon black particles having an average particle size of less thanabout 3.0 microns in said dispersion;

(2) an effective dispersing amount of a surfactant which is compatiblewith said carbon black; and

(3) a liquid dispersing medium, wherein the amount of carbon black issufficient to coat substantially all of said non-conducting surface andis less than about 4% by weight of said liquid dispersion;

(b) separating substantially all of the liquid dispersing medium fromsaid applied dispersion, thereby depositing said carbon black particlesin a substantially continuous layer on said non-conducting portions ofsaid hole walls;

(c) microetching said metal layers of said printed wiring board toremove deposited carbon black therefrom; and

(d) electroplating a substantially continuous metal layer over thedeposited carbon black layer on said non-conducting portions of holewalls, thereby electrically connecting said metal layers of said printedwiring board;

wherein said improvement comprises:

contacting said deposited carbon black particles after step (b) andbefore step (c) with aqueous solution of an alkali metal borate.

These printed wiring boards are usually comprised of an epoxyresin/glass fiber mixture positioned between two conductive metal layers(e.g. copper or nickel plates or foils) or a multiplicity of saidalternating layers. Applying a conducting metal layer over saidnon-conducting portions of said through hole walls electrically connectsthe conductive metal layers.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is directed toward an improvement to theabove-noted Minten and Pismennaya process for preparing the through holewalls of a printed wiring board for the application of an electroplatedlayer of copper over a non-conducting layer separating two plates orfoils of copper.

This preparation process entails placing a selected liquid carbon blackdispersion over the non-conducting portions of the through hole wallsbefore electroplating. The liquid carbon black dispersion is a completereplacement for the electroless copper bath and all of its attendantprocess chemistry. That is, it replaces the preactivator step, the Pd/Snactivator, the accelerator step and the electroless bath itself. Afterthe dispersion medium has been removed and the carbon black particleshave been deposited on the through hole walls, the critical feature ofthe present invention contemplates contacting (e.g. immersing) anaqueous alkaline solution of an alkali metal borate to facilitate theremoval of any loosely held on carbon black particles. This step workstogether with the standard microetch step to more effectively removecarbon black particles from the metal surfaces of the PWB and make theremaining carbon black surfaces on the non-conductive portions of thePWB very smooth for electroplating.

Printed wiring boards (also known as printed circuit boards) aregenerally laminated materials comprised of two or more plates or foilsof copper, which are separated from each other by a layer ofnon-conducting material. Although copper is generally used as theelectroplating metal in printed wiring boards, those skilled in the artwill recognize that other metals such as nickel, gold, palladium, silverand the like can also be electroplated by the process of this invention.The non-conducting layer or layers are preferably an organic materialsuch as an epoxy resin impregnated with glass fiber particles. However,the non-conducting layer may also be comprised of thermosetting resins,thermoplastic resins, and mixtures thereof, with or without reinforcingmaterials such as fiberglass and fillers.

Suitable thermoplastic resins include the acetal resins; acrylics, suchas methyl acrylate; cellulosic resins, such as ethyl cellulose,cellulose acetate, cellulose propionate, cellulose acetate butyrate,cellulose nitrate, and the like; chlorinated polyethers; nylon;polyethylene; polypropylene; polystyrene; styrene blends, such asacrylonitrile styrene co-polymers and acrylonitrile-butadiene-styrene(ABS) co-polymers; polycarbonates; polychlorotrifluoroethylene; andvinyl polymers and co-polymers, such as vinyl acetate, vinyl alcohol,vinyl butyral, vinyl chloride, vinyl chloride-acetate co-polymer,vinylidene chloride and vinyl formal; and the like.

Suitable thermosetting resins include alkyl phthalate, furane;melamine-formaldehyde; phenol formaldehyde and phenol-furfuralco-polymers; alone or compounded with butadiene acrylonitrile co-polymeror acrylonitrile-butadiene-styrene (ABS) co-polymers; polyacrylicesters; silicones; urea formaldehydes; epoxy resins; polyimides; alkylresins; glyceryl phthalates; polyesters; and the like.

In many printed wiring board designs, the electrical pathway or patternrequires a connection between the separated copper plates at certainpoints in the pattern. This is usually accomplished by drilling holes atthe desired locations through the laminate of copper plates and thenon-conducting layer and then connecting the separate metal plates. Thehole diameters of printed wiring boards generally range from betweenabout 0.5 and about 10 millimeters in diameter, and preferably fromabout 1 to about 5 millimeters.

After drilling these through holes, it may be desirable to deburr theholes to make the hole walls relatively smooth. In the case ofmultilayer printed wiring boards, it may also be desirable to subjectthe boards to a desmear or etchback operations to clean the inner copperinterfacing surfaces of the through holes. Suitable preparativeoperations include any or all of the presently available conventionaloperations including conventional permanganate desmearing processes.

Once the surfaces of through holes have been made relatively smooth forplating, it is preferred to subject the PWB to a precleaning process inorder to place the printed wiring board in condition for receiving theliquid carbon black dispersion. In one preferred pre-cleaning operation,the printed wiring board is first placed in a cleaner/conditioner bathfor about 1 to 10 minutes at a temperature of about 45° C. to about 70°C. to remove grease and other impurities from the hole wall surfaces. Inthis embodiment, one preferred cleaner is comprised of monoethanolamine,a nonionic surfactant and ethylene glycol in water; which is availableas "BLACKHOLE Cleaner 2" from the Olin Hunt Specialty Products, Inc. ofWest Paterson, N.J.

After the application of the cleaner, the PWB is subsequently rinsed inwater to remove excess cleaner from the board and then contacted with aconditioner solution. The preferred method of contacting with aconditioner is dipping the cleaned PWB into a room temperature aqueousconditioner bath for about 1-10 minutes. This conditioner solution isused to ensure that substantially all of the hole wall glass/epoxysurfaces are properly prepared to accept a continuous layer of thesubsequent carbon black particles. Such conditioner solutions have beencustomarily used in electroless processing to precondition the boardsfor the electroless chemistry. See U.S. Pat. No. 4,634,691, which issuedto Lindsey on Jan. 6, 1987, for a discussion of conditioner solution.The Lindsey patent is incorporated herein by reference in its entirety.One preferred conditioner is "BLACKHOLE Conditioner" available from OlinHunt Specialty Products, Inc. of West Paterson, N.J. This conditionerformulation comprises the miture of monoethanolamine and a polyamineresin in water and has a pH of about 10. The preferred concentration oftotal conditioner ingredients in water is from about 1% to about 10% byweight.

If the cleaner solution precedes the aqueous cleaner solution, it ispreferred to rinse the treated board in a water rinse between thecleaner solution and the conditioner solution. It is also preferred touse a water rinse bath after the conditioner treatment.

The liquid carbon black dispersion is next applied to or contacted withthe conditioned PWB. This dispersion contains three criticalingredients, namely, carbon black, one or more surfactants capable ofdispersing the carbon black and a liquid dispersing medium such aswater. The preferred method of applying the dispersion to the PWBinclude immersion, spraying or other methods of applying chemicals usedin the PWB industry. A single working bath is sufficient for applyingthis carbon black dispersion; however, more than one bath may be usedfor rework or other purposes.

In preparing this liquid dispersion, the three critical ingredients andany other preferred ingredients are thoroughly mixed together to form astable dispersion. This may be accomplished by subjecting aconcentratedform of the dispersion to ball milling, colloidal milling, high-shearmilling or ultrasonic techniques. The thoroughly mixed dispersion islater diluted with more water while agitating to the desiredconcentration for the working bath. The preferred method of mixing isball milling a concentrated form of the dispersion in a container havingglass mineral or plastic beads therein for a period of about 1 to about24 hours. This thorough mixing allows for the carbon black particles tobe intimately coated or wetted with the surfactant. This mixedconcentrate is then mixed with water or some other liquid dispersingmedium to the desired concentration. The working bath is preferably keptagitated during both the diluting and applying steps to aid in keepingthe dispersion stable.

As stated above, the carbon black particles should have an averageparticle diameter below about 3 microns while in the dispersion. It isdesirable to have this average particle diameter of carbon black assmall as possible to obtain the desired plating characteristics ofsubstantially even plating and no plating pullaways. It is preferredthat the carbon black particles have an average particle diameter fromabout 0.1 to about 3.0, more preferably from 0.2 and about 2.0, micronswhen in said dispersion. The term "average particle diameter" asemployed herein in both the specification and claims refers to averagemean diameter of the particles (the average by number). The average meandiameter in the dispersion may be determined through the use of either aNiComp Model 270 submicron particle sizer (Version 3.0) or a HIAC PA-720automatic particle size analyzer (both available from the HIAC/ROYCOInstrument Division of Pacific Scientific of Menlo Park, CA).

All types of carbon blacks may be used for this invention including thecommonly available furnace blacks. However, it is preferred to utilizecarbon blacks which are initially acidic or neutral, i.e. those whichhave a pH of between about 1 and about 7.5 and more preferably betweenabout 2 and about 4 when slurried with water. Carbon black particles ofthe preferred type contain between about 1 and about 10 percent byweight of volatiles and have an amorphous structure.

These preferred carbon black particles are also very porous andgenerally their surface areas are from between about 45 to about 1100,and preferably between about 300 and about 600, square meters per gramby the BET method (method of Brunauer-Emmett-Teller).

Illustrative carbon blacks suitable for use of this invention includeCabot XC-72R Conductive, Cabot Monarch 800, Carbot Monarch 1300, allmanufactured by Cabot Corporation of Boston, MA. Other suitable carbonblacks include Columbian T-10189, Columbian Conductex 975 Conductive,Columbian CC-40-220, and Columbian Raven 3500, all available fromColumbian Carbon Company of New York, NY. Monarch 800 and Raven 3500 arethe two most preferred carbon blacks because of their ease of dispersionand low pH.

The term "liquid dispersing medium" as used herein in the presentspecification and claims includes water and polar organic solvents (bothprotic and aprotic). Suitable protic polar organic solvents may includelower alcohols (C₁ -C₄) such as methanol, ethanol, isopropanol andisobutanol; polyhydric alcohols such as glycols (i.e. triethyleneglycols); ether-alcohols such as cellosolve; organic acids, such asformic acid and acetic acid; acid derivatives such as trichloroaceticacid; and sulfonic acids such as methane sulfonic acid. Suitable aproticpolar organic solvents include aldehydes such as acetaldehyde; ketonessuch as acetone; aprotic aromatic solvents such as toluene and mineralspirits; aprotic halogenated hydrocarbons such as dichlorofluoromethaneand dichlorodifluoromethane (FREON); dimethylformamide (DMF);N-methylpyrrolidone; dimethylsulfoxide (DMSO); and esters of carboxylicacids such as methylformate, ethylacetate and cellosolve acetate. Thepreferred liquid dispersing medium is water because of cost and ease ofuse considerations. It is preferred to utilize deionized water which isfree of lime, fluorine, iodine and other impurities normally found intap water, in order to minimize interference of foreign ions during thesubsequent electroplating step.

In addition to the water and carbon black, a third critical ingredientis needed in the dispersion, namely, a surfactant capable of dispersingsaid carbon black in said liquid dispersing medium (i.e. compatible withsaid carbon black and liquid dispersing medium). One or more of these isadded to the dispersion in order to enhance the wetting ability andstability of the carbon black and permit maximum penetration by thecarbon black within the pores and fibers of the non-conducting layer.Suitable wetting agents include anionic, nonionic and cationicsurfactants (or combinations thereof such as amphoteric surfactants).The surfactants should be soluble, stable and preferably nonfoaming inthe liquid carbon black dispersion. In general, for a polar continuousphase as in water, the surfactants should preferably have a high HLBnumber (8-18). The preferred type of surfactant will depend mainly onthe pH of the dispersion. If the total dispersion is alkaline (i.e. hasan overall pH in the basic range), it is preferred to employ an anionicor nonionic surfactant. Acceptable anionic surfactants include sodium orpotassium salts of naphthalene sulfonic acid such as DARVAN No. 1 (R. T.Vanderbilt Co.), ECCOWET LF (Eastern Color and Chemical), PETRO AA,PETRO ULF (Petro Chemical Co., Inc.), and AEROSOL OT (AmericanCyanamid). Preferred anionic surfactants include neutralized phosphateester-type surfactants such as MAPHOS 55, 56, 8135, 60A, L6 (MazerChemicals Inc.). The most preferable anionic surfactant for a liquidcarbon black dispersion is MAPHOS 56. Suitable nonionic surfactantsinclude ethoxylated nonyl phenols such as POLY-TERGENT B-series (OlinCorporation) or alkoxylated linear alcohols such as POLY-TERGENTSL-series (Olin Corporation).

If the total dispersion is acidic, it is preferred to employ selectedanionic surfactants or cationic surfactants. An acceptable group ofanionic surfactants would be the sodium or potassium salts ofnaphthalene sulfonic acid described above. Acceptable cationicsurfactants include cetyl dimethyl benzyl ammonium chloride such asAMMONYX T (Onyx Chemical Corporation); an ethanolated alkylguanidineamine complex such as AEROSOL C-61 (American Cyanamid); lipocals;dodecyldiphenyl oxide disulfonic acid (DDODA) such as DOWFAX 2A1 (DowChemical); a sodium salt of DDODA such as STRODEX (Dexter ChemicalCorporation); and salts of complex organic phosphate esters. Preferredsurfactants include amphoteric potassium salts of a complex amino acidbased on fatty amines such as MAFO 13 and cationic ethoxylated soyaamines such as MAZEEN S-5 or MAZTREAT (Mazer Chemicals Inc.).Combinations of surfactants may be employed.

The amount of carbon black in the dispersion should be less than about4% by weight of the dispersion, preferably, less than about 2% byweight. It has been found that the use of higher concentrations ofcarbon blacks provide undesirable plating characteristics. In the sameregard, the solids content (i.e. all of the ingredients other than theliquid dispersing medium) is preferably less than 10% by weight of thedispersion, more preferably, less than about 5.6% by weight.

The three above-noted critical components of the present invention,namely, the carbon black, liquid dispersing medium and surfactant, maybe employed alone to form a liquid dispersion. In some situations, itmay be desirable to add other preferred ingredients to this dispersion.

One additional preferred component of the liquid carbon black-containingdispersion is a strong basic material such as an alkaline hydroxide.Suitable strong basic materials include alkali metal hydroxides such aspotassium hydroxide, sodium hydroxide, and lithium hydroxide. Ammoniumhydroxide or alkaline earth metal hydroxides such as calcium hydroxidemay also be employed, if desired. Potassium hydroxide is the mostpreferred strong basic material. The term "alkaline hydroxide" is usedthroughout the description and claims to identify these strong basicmaterials used in both this step and the conditioning step. Sufficientalkaline hydroxide may be added to the liquid carbon black dispersion ina proportion sufficient to increase the pH of the resulting carbonblack-containing dispersion to between about 10 and about 14, andpreferably between about 10 and about 12.

Following is a typical formulation of a suitable aqueous alkalinedispersion of carbon black showing the general range of proportions aswell as the preferred range of proportions for the various components:

    ______________________________________                                        Component     General Range                                                                             Preferred Range                                     ______________________________________                                        Carbon Black  0.1-4% by wt.                                                                             0.2-2% by wt                                        Surfactant    0.01-4%     0.05-2%                                             Alkaline Hydroxide                                                                          0-1%        0.4-0.8%                                            Water         balance     balance                                             ______________________________________                                    

The liquid dispersion of carbon black is typically placed in a suitableagitated vessel and the printed wiring board to be treated is immersedin, sprayed with or otherwise contacted with the liquid dispersion. Thetemperature of the liquid dispersion in an immersion bath is maintainedin the range of between about 15° C. and about 35° C. and preferablybetween about 20° C. and about 30° C., while the conditioned printedwiring board is immersed therein. The period of immersion generallyranges from about 1 to about 10, and preferably from about 3 and about 5minutes. During immersion, the liquid carbon black-containing dispersionpenetrates the holes of the printed wiring board and wets and contactsthe glass fiber as well as the epoxy resin which forms the components ofthe insulating layer. The immersed board is then removed from the bathof the liquid carbon black-containing dispersion.

The carbon black-covered board is then subjected to a step wheresubstantially all (i.e. over about 95% by weight) of the water in theapplied dispersion is removed and a dried deposit containing carbonblack is left in the holes and on other exposed surfaces of thenon-conducting layer. This may be accomplished by several methods suchas by evaporation at room temperature, by a vacuum, or by heating theboard for a short time at an elevated temperature, or by otherequivalent means. Heating at an elevated temperature is the preferredmethod. Heating is generally carried out for between about 5 and about45 minutes at a temperature of from about 75° C. to about 120° C., morepreferably from about 80° C. to 98° C. To insure complete coverage ofthe hole walls, the procedure of immersing the board in the liquidcarbon black dispersion and then drying may be repeated one or moretimes.

This drying step yields a board which may be completely coated with thecarbon black dispersion. This dispersion is not only coated on thedrilled hole surfaces, which is desirable, but also entirely coats thecopper plate or foil surfaces which is undesirable. Thus prior to anyphotoimaging process all carbon black must be removed from the copperplate or foil surface.

As a critical feature of the present invention, the dried deposit ofcarbon black in the through holes is then contacted with an aqueousalkaline solution containing a alkali metal borate. The preferred alkalimetal borate is sodium borate. The preferred pH range of this alkalinesolution is from about 9.5 to about 11.5. The preferred bath temperatureis from about 20° C. to about 50° C. The functions of this step includeremoving excess carbon black material on the rims and inner metal wallsof the PWB through holes and remove any loose carbon black particlesfrom the through hole walls which might cause an undesirable unevenplated surface to result. The alkali metal borate also increases thesurface porosity of the carbon black.

The amount of alkali borate should be sufficient to remove substantiallyall of the loose or easily removable carbon black particles from theareas of the through holes. The preferred concentration may vary fromabout 2 to about 50 grams per liter of water employed. This contactingstep may be carried out by placing the PWB in an aqueous bath containingthe alkali metal borate at a temperature from about 20° C. to about 50°C. for about 20 seconds to about 5 minutes.

The further removal of the carbon black, specifically from the outercopper surfaces including, especially, the rims of the drilled holeswhile leaving the coating intact on the glass fibers and epoxy surfaceof the hole walls, may be achieved by the employment of a mechanicallyscrubbing operation or a microetch or both. The microetch is preferredbecause of ease of use. One suitable sodium persulfate microetch is"BLACKHOLE MICROCLEAN I" available from Olin Hunt Specialty ProductsInc. and referred to above. This product is preferrably combined withsufficient sulfuric acid to make a microetch bath containing 100-300grams of sodium persulfate per liter and 0.5%-5% by volume sulfuricacid. The mechanism by which this microetch works is by not attackingthe carbon black material deposited on the copper foil directly, butrather to attack exclusively the first few atomic layers of copperdirectly below which provides the adhesion for the coating. Hence, thefully coated board is immersed in the microetch solution to "flake" offthe carbon black from the copper surfaces into solution in the form ofmicro-flakelets. These micro-flakelets are removed from the microetchbath either by filtration through a pump or via a weir type filterarrangement commonly used in the PWB industry. The liquid carbon blackdispersion, the alkali metal borate treatment, the microetch treatment,and the intermittent water rinses are preferably carried out byimmersing the PWB in a bath constructed of polypropylene or polyvinylchloride (PVC) and kept agitated by a recirculation pump or pumped inair.

In the case of a multilayer type board this combination of employing aalkali borate solution followed by a microetching step is especiallyadvantageous. This procedure performs two very desirable tasks:

A. It removes substantially all excess carbon black material adhering tothe outer copper plates or foils, the rims of the through holes and theexposed surfaces of copper inner plates or foils in multilayer PWBs.

B. It chemically cleans and microetches slightly the outer coppersurfaces, thereby making them excellent bases for either dry filmapplication or the electrolytic deposition of copper when followed bymechanically scrubbing the PWB.

After the microetch step and a subsequent optional water rinse, the PWBmay now either proceed to the photoimaging process and later beelectroplated or be directly panel electroplated.

The thus treated printed wiring board is then immersed in a suitableelectroplating bath for applying a copper coating on the hole walls ofthe non-conducting layer.

The present invention contemplates the use of any and all electroplatingoperations conventionally employed in applying a metal layer to thethrough hole walls of a PWB. Therefore this claimed invention should notbe limited to any particular electroplating bath parameters.

A typical copper electroplating bath is comprised of the followingcomponents in the following proportions:

    ______________________________________                                                      General      Preferred                                          Component     Proportions  Proportions                                        ______________________________________                                        Copper (as metal)                                                                           2-3      oz/gal  2.25-2.75                                                                            oz/gal                                  Copper Sulfate                                                                              5-10     oz/gal  6-9    oz/gal                                  98% Concentrated                                                                            23-32    oz/gal  27-30  oz/gal                                  H.sub.2 SO.sub.4 (by weight)                                                  Chloride Ion  20-100   mg/1    40-60  mg/l                                    ______________________________________                                    

The electroplating bath is normally agitated and preferably maintainedat a temperature of between about 20° C. and about 25° C. Theelectroplating bath is provided with anodes, generally constructed ofcopper, and the printed wiring board to be plated is connected as acathode to the electroplating circuit. For example, a current of about30 amps per square foot is impressed across the electroplating circuitfor a period of between about 40 and about 60 minutes in order to effectcopper plating on the hole walls of the non-conducting layer positionedbetween the two plates of copper up to a thickness of about 1 mil ±0.2mil. This copper plating of the hole wall provided a current pathbetween the copper layers of the printed wiring board. Other suitableelectroplating conditions may be employed, if desired. Otherelectroplating bath compositions containing other copper salts or othermetal salts such as salts of nickel, gold, palladium, silver and thelike may be employed, if desired.

The printed wiring board is removed from the copper electroplating bathand then washed and dried to provide a board which is further processedby applying photoresist compounds and the like, as is known in the artfor the preparation of printed wiring boards.

It has been found that, even with excess dwell time in the liquid carbonblack bath, the resulting carbon black coating does not appreciablyincrease in thickness. This seems to mean that this is a surfaceadsorption process and once coverage is attained over the entire surfaceof the hole profile, no more build up of material will take place.

The following example is presented to define the invention more fullywithout any intention of being limited thereby. All parts andpercentages are by weight unless otherwise explicitly specified.

PRINTED WIRING CIRCUIT BOARD SPECIFICATIONS

One double-sided and one multilayer control laminated printed wiringboard and four double-sided and four multilayer test printed wiringboards were treated by the process of this invention. These multilayerprinted wiring boards were comprised of four 35 micron thick copperplates secured by pressure fusing them with epoxy resin/glass fiberlayers in an alternative fashion. The double-sided printed wiring boardswere comprised of two 35 micron thick copper plates secured by pressurefusing to the opposite sides of an epoxy resin/glass fiber layer. Thesedouble-sided and multilayer printed wiring boards were about 15.24centimeters wide and about 22.86 centimeters in length. There were about500 to about 1000 holes, each about 1.0 millimeters in diameter drilledthrough the copper plates and expoxy resin/glass fiber layer.

EXAMPLE 1

These double-sided printed wiring boards described above were preparedfor copper electroplating their through holes by first mechanicallyscrubbing the surfaces of the board. These multilayer printed wiringboards described above were prepared for copper electroplating by firstcontacting said printed wiring boards with standard permanganate desmearsolutions such as the Permolin system from Olin Hunt Specialty ProductsInc. of West Paterson, N.J. The boards were then immersed in thefollowing sequence of aqueous baths (each about 132 liters volume) forthe indicated times:

1. Cleaner (5 minutes)

2. Rinse with tap water (2 minutes)

3. Conditioner (4 minutes)

4. Rinse with tap water (2 minutes)

5. Carbon black preplating dispersion (4 minutes) [then dry at 93° C.(20 minutes)]

6. Sodium borate premicroetch conditioner (30 seconds)

7. Rinse with tap water (2 minutes)

8. Sodium persulfate microetch (30 seconds)

9. Rinse with tap water (20 seconds)

10. Anti-tarnish solution (20 seconds)

11. Rinse with tap water (20 seconds) Bath 1 was an aqueous solutioncontaining a cleaner formulation comprised of monoethanolamine, anonionic surfactant and ethylene glycol in water to remove grease andother impurities from the hole wall surfaces of the board. The bath washeated to about 60° C. to facilitate this cleaning. The cleanerformulation is available as "Blackhole Cleaner 2" from Olin HuntSpecialty Products Inc. of West Paterson, N.J.

Bath 3 was a room temperature aqueous alkaline bath which containsmonoethanolamine and a polyelectrolyte polyamide amine copolymer and hasa pH of about 10 to condition the hole wall surfaces of the board. Theconditioner formulation is available as "Blackhole Conditioner" fromOlin Hunt Specialty Products Inc. of West Paterson, N.J.

Bath 5 was a room temperature deionized water bath containing the carbonblack preplating formulation. In this bath, the proportions of eachingredient were as follows:

0.094 by weight anionic surfactant

0.600 by weight KOH

0.310 by weight carbon black

1.004 by weight solids

The balance of the bath was deionized water. This carbon blackdispersion of bath 5 was prepared by milling a concentrated form of thisdispersion in a pebble mill containing stone pebbles so that theconcentration of pebbles occupied about one third of the mill volume.The surfactant was dissolved in deionized water/KOH to give a continuousphase. Then the carbon black was added. Milling time was six hours.After milling, the concentrate was diluted with sufficient deionizedwater to make the dispersion in the above indicated proportions.

After bath 5, the boards were placed in a hot air recirculatory oven andheated to 93° C. for 20 minutes. This drying step removed the water fromthe carbon black coating on the board, thereby leaving a dried depositof carbon black all over the board and in the through holes of theboard. The drying promotes adhesion between the carbon black and thenon-conductive surfaces of the board.

Bath 6 was an aqueous bath heated to 43° C. and contained 10 grams ofsodium tetraborate decahydrate and about 5.2 grams of liquid causticsoda per liter of deionized water to adjust the pH to 10.5. Its functionwas to facilitate the removal of dried carbon black material from therim of the through holes as well as other copper surfaces of the boardsin the following sodium persulfate/H₂ SO₄ microetch step.

Bath 8 was a room temperature aqueous bath and contained 200 grams of asodium persulfate per liter of deionized water and 0.5% by volume ofconcentrated H₂ SO₄. Its function was to microetch the copper surfacesof the board so as to remove the deposited carbon black from thesurfaces. It does not act on the resin/glass surfaces. This sodiumpersulfate microetch was made from "Blackhole™ Microclean I" and isavailable from Olin Hunt Specialty Products, Inc. of West Paterson, N.J.

Bath 10 was a room temperature aqueous bath and contained 50 grams ofcitric acid per liter of deionized water and 0.5% by volume ofconcentrated H₂ SO₄. Its function was to prevent the copper surfaces ofthe printed wiring boards from tarnishing.

Rinse baths 2, 4, 7, 9 and 11 were employed to prevent the carryover ofchemicals from one treatment bath into the next.

After treatment with this sequence of baths, the printed wiring boardswere placed in an electroplating bath provided with agitation means andheating means and which contained an electrolyte bath comprised of thefollowing:

Plating Bath Composition

    ______________________________________                                        Plating Bath Composition                                                      Component             Proportion                                              ______________________________________                                        Copper (as metal)     2.5   oz./gal.                                          Copper Sulfate        6.2   oz./gal.                                          98% Concentrated H.sub.2 SO.sub.4                                                                   30    oz/gal.                                           (by weight)                                                                   Chloride ion          40    mg/l                                              ______________________________________                                    

The printed wiring board was connected as a cathode in theelectroplating vessel having a volume of about 720 liters. Twelve copperbars were immersed in the electrolyte and connected to the cell circuitsas anodes. The copper bars had a length of about 91 cm; a width of about9 cm and a thickness of about 4 cm. Each face was about 819 square cm. Adirect current of 30 amps per square foot was impressed across theelectrodes in the electroplating bath for approximately 55 minutes. Thebath was maintained at a temperature of about 25° C. during this periodand agitation was effected by air sparging. At the end of this period,the printed wiring board was disconnected from the electroplating crcuitremoved from the electrolyte, washed with tap water and dried.

An examination of the through holes of the resulting electroplatedprinted wiring boards was conducted and the following parameters wereevaluated:

Corner contamination--residual dried carbon black dispersion material onthe rims of the through holes.

Pullaway--adhesion failure of the plated copper to the hole wall.

Epoxy voids--absence of plated copper on the resin surfaces.

Glass voids--absence of plated copper on the glass surfaces.

Rim voids--absence of plated copper at or just below the rim of thethrough hole

Innerlayer contamination--(for multilayer boards only) residualblackhole material on the interconnect in the through hole.

A rating system for these defects was devised by assigning numbers from1 to 4 depending on the severity of the defect; 1 indicating no defectwas observed and 4 indicating the defect was observed on essentially allof the through holes inspected.

32 cross-sections with 5 through holes per cross-section totalling 160through holes were evaluated each for the control and the abovedescribed example.

The average results of these evaluations for the control and thisinvention are listed in Tables 1 and 2 below. The pullaway rating wasafter thermal shock testing in which the samples were floated in amolten solder bath at 550° F. for 10 seconds.

                  TABLE 1                                                         ______________________________________                                        AVERAGE DEFECT RATING FOR CONTROL                                                    Inner-                                                                        layer Corner                                                                  Fold  Contami- Pull   Glass Epoxy Rim                                         Voids nation   away   Voids Voids Voids                                ______________________________________                                        Double-sided                                                                           --      2.5      1.5  1.0   1.0   1.0                                Multilayer                                                                             1.0     3.4      1.5  1.0   1.0   1.0                                ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        AVERAGE DEFECT RATING FOR SODIUM BORATE                                       BATH                                                                                 Inner-                                                                        layer Corner                                                                  Fold  Contami- Pull   Glass Epoxy Rim                                         Voids nation   Away   Voids Voids Voids                                ______________________________________                                        Double-sided                                                                           --      1.0      1.5  1.0   1.0   1.0                                Multilayer                                                                             1.0     1.0      1.5  1.0   1.0   1.0                                ______________________________________                                    

It is clearly evident from this data that incorporation of the sodiumborate bath prior to the Microclean I bath eliminated the undesirabledefect known as corner contamin,ation while not adversely effecting anyof the other critical parameters. Also examination of the resultingelectroplated boards showed that the hole walls were coated with arelatively uniform layer of copper (1.0 mils ±0.2 mils) and no "dogboning" was observed (i.e. this latter defect is an undersirablecondition where the plated layer is thicker near the copper laminateareas of the PWB.

EXAMPLE 2

The double-sided printed wiring boards described above were prepared forcopper electroplating their through holes by first mechanical scrubbingthe surfaces of the board. The multilayer printed wiring boardsdescribed above were prepared for copper electroplating by firstcontacting said printed wiring board with standard permanganate desmearsolutions (i.e. as the Permolin system from Olin Hunt Specialty ProductsInc. of West Paterson, N.J.). The boards were then immersed in thefollowing sequence of aqueous baths (each about 132 liters volume) forthe indicated times:

1. Cleaner (5 minutes)

2. Rinse with tap water (2 minutes)

3. Conditioner (4 minutes)

4. Rinse with tap water (2 minutes)

5. Carbon black preplating dispersion (4 minutes) [then dry at 93° C.(20 minutes)]

6. Sodium borate premicroetch conditioner (30 seconds)

7. Rinse with tap water (2 minutes)

8. Sodium persulfate microetch (30 seconds)

9. Rinse with tap water (20 seconds)

10. Anti-tarnish solution (20 seconds)

11. Rinse with tap water (20 seconds)

In the case of the control printed wiring boards, bath 6 and bath 7 werenot used. Bath 1 was an aqueous solution containing a cleanerformulation comprised of monoethanolamine, a nonionic surfactant andethylene glycol in water to remove grease and other impurities from thehole wall surfaces of the board. The bath was heated to about 60° C. tofacilitate this cleaning. The cleaner formulation is available as"Blackhole Cleaner 2" from Olin Hunt Specialty Products Inc. of WestPaterson, N.J.

Bath 3 was a room temperature aqueous bath which containedmonoethanolamine and a polyelectrolyte to condition the hole wallsurfaces of the board. The conditioner formulation is available as"Blackhole Conditioner" from Olin Hunt Specialty Products Inc. of WestPaterson, N.J.

Bath 5 was a room temperature deionized water bath containing the carbonblack preplating formulation. In this bath, the proportions of eachingredient were as follows:

0.094 by weight anionic surfactant

0.600 by weight KOH

0.310 by weight carbon black

1.004 by weight solids

The balance of the bath was deionized water. This carbon blackdispersion of bath 5 was prepared by milling a concentrated form of thisdispersion in a pebble mill containing stone pebbles so that theconentration of pebbles occupied about one third of the mill volume. Thesurfactant was dissolved in deionized water/KOH to give a continuousphase. Then the carbon black was added. Milling time was six hours.After milling, the concentrate was diluted with sufficient deionizedwater to make the dispersion in the above indicated proportions.

After bath 5, the boards were placed in a hot air recirculatory oven andheated to 93° C. for 20 minutes. This drying step removed the water fromthe carbon black coating on the board, thereby leaving a dried depositof carbon black all over the board and in the through holes of theboard. The drying promotes adhesion between the carbon black and thenon-conductive surfaces of the board.

Bath 7 as an aqueous bath heated to 43° C. and contained 10 grams ofsodium tetraborate decahydrate and about 5.2 grams of liquid causticsoda per liter of deionized water to adjust the pH to 10.5. The bath wasused in a conveyorized spray mode at a spray pressure of 2 psig insteadof the standard dip application. Its function was to facilitate theremoval of dried carbon black material from the rim of the through holesas well as other copper surfaces of the boards in the following sodiumpersulface/H₂ SO₄ microtech step.

Bath 8 was a sodium persulfate solution in deionized water and 0.5% byvolume of concentrated H₂ SO₄. Its function was to microetch the coppersurfaces of the board so as to remove the deposited carbon black fromthe surfaces. It does not act on the resin/glass surfaces. This sodiumpersulface microetch was made from "Blackhole™ Microclean I" and isavailable from Olin Hunt Specialty Products Inc. of West Paterson, N.J.

Bath 10 was a room temperature aqueous bath and contained 50 grams ofcitric acid per liter of deionized water and 0.5% by volume ofconcentrated H₂ SO₄ . Its function was to prevent the copper surfaces ofthe printed wiring boards from tarnishing.

Rinse baths 2, 4, 7, 9 and 11 were employed to prevent the carryover ofchemicals from one treatment bath into the next.

After treatment with this sequence of baths, the printed wiring boardswere placed in an electroplating bath provided with agitation means andheating means and which contained an electrolyte bath comprised of thefollowing:

Plating Bath Composition

    ______________________________________                                        Plating Bath Composition                                                      Component             Proportion                                              ______________________________________                                        Copper (as metal)     2.5   oz./gal.                                          Copper Sulfate        6.2   oz./gal.                                          98% Concentrated H.sub.2 SO.sub.4                                                                   30    oz/gal.                                           (by weight)                                                                   Chloride ion          40    mg/l                                              ______________________________________                                    

The printed wiring board was connected as a cathode in theelectroplating vessel having a volume of about 720 liters. Twelve copperbars were immersed in the electrolyte and connected to the cell circuitsas anodes. The copper bars had a length of about 91 cm; a width of about9 cm and a thickness of about 4 cm. Each face was about 819 square cm. Adirect current of 30 amps per square foot was impressed across theelectrodes in the electroplating bath for approximately 55 minutes. Thebath was maintained at a temperature of about 25° C. during this periodand agitation was effected by air sparging. At the end of this period,the printed wiring board was disconnected from the electroplatingcircuit removed from the electrolyte, washed with tap water and dried.

An examination of the through holes of the resulting electroplatedprinted wiring boards was conducted and the following parameters wereevaluated:

Corner contamination--residual dried carbon black dispersion material onthe rims of the through holes.

Pullaway--adhesion failure of the plated copper to the hole wall.

Epoxy voids--absence of plated copper on the resin surfaces.

Glass voids--absence of plated copper on the glass surfaces.

Rim voids--absence of plated copper at or just below the rim of thethrough hole

Innerlayer contamination--(for multilayer boards only) residualblackhole material on the interconnect in the through hole.

A rating system for these defects was devised by assigning numbers from1 to 4 depending on the severity of the defect; 1 indicating no defectwas observed and 4 indicating the defect was observed on essentially allof the through holes inspected.

32 cross-sections with 5 through holes per cross-section totalling 160through holes were evaluated each for the control and the abovedescribed example.

The average results of these evaluations for the control and thisinvention are listed in Tables 3 and 4 below. The pullaway rating wasafter thermal shock testing in which the sample was floated in a moltensolder bath at 550° F. for 10 seconds.

It is clearly evident from this data that incorporation of the sodiumborate bath prior to the Microclean I bath eliminated the undesirabledefect known as corner contamination while not adversely effecting anyof the other critical parameters.

                  TABLE 3                                                         ______________________________________                                        AVERAGE DEFECT RATING FOR CONTROL                                                    Inner-                                                                        layer Corner                                                                  Fold  Contami- Pull   Glass Epoxy Rim                                         Voids nation   Away   Voids Voids Voids                                ______________________________________                                        Double-sided                                                                           --      2.5      1.5  1.0   1.0   1.0                                Multilayer                                                                             1.0     3.4      1.5  1.0   1.0   1.0                                ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        AVERAGE DEFECT RATING FOR SODIUM BORATE                                       BATH                                                                                 Inner-                                                                        layer Corner                                                                  Fold  Contami- Pull   Glass Epoxy Rim                                         Voids nation   Away   Voids Voids Voids                                ______________________________________                                        Double-sided                                                                           --      1.0      1.5  1.0   1.0   1.0                                Multilayer                                                                             1.0     1.0      1.1  1.0   1.0   1.0                                ______________________________________                                    

also examination of the resulting electroplated boards showed that thehole walls were coated with a relatively uniform layer of copper (1.0mils ±0.2 mils) and no "dog boning" was observed (i.e. this latterdefect is an undersirable condition where the plated layer is thickernear the copper laminate areas of the PWB.

What is claimed is:
 1. In the process for electroplating the walls ofthrough holes in a laminated printed wiring board comprised of at leastone non-conducting layer laminated to at least two separate conductivemetal layers, which comprises the steps:(a) contacting said printedwiring board having said through holes with a liquid dispersion ofcarbon black comprised of:(1) carbon black particles having an averageparticle size of less than about 3.0 microns in said dispersion; (2) aneffective dispersing amount of a surfactant which is compatible withsaid carbon black; and (3) a liquid dispersing medium, wherein theamount of carbon black is sufficient to coat substantially all of saidnon-conducting surfaces and is less than about 4% by weight of saidliquid dispersion; (b) separating substantially all of the liquiddispersing medium from said applied dispersion, thereby depositing saidcarbon black particles in a substantially continuous layer on saidnon-conducting portions of said hole walls; (c) microetching said metallayers of said printed wiring board to remove deposited carbon blacktherefrom; and (d) later electroplating a substantially continuous metallayer over the deposited carbon black layer on said non-conductingportions of hole walls, thereby electrically connecting said metallayers of said printed wiring board;wherein said improvement comprises:contacting said deposited carbon black particles after step (b) andbefore step (c) with aqueous alkaline solution of an alkal metal borate.2. The process of claim 1 wherein said aqueous alkaline solution is amixture of an alkaline metal hydroxide and an alkali metal borate. 3.The process of claim 1 wherein said alkaline borate is sodium borate. 4.The process of claim 1 wherein said contacting step of the improvementis carried out by immersing the printed wiring board in an aqueousalkaline bath containing said alkali metal borate.
 5. The process ofclaim 1 wherein said contacting step of the improvement occurs at atemperature from about 30° C. to about 60° C.
 6. The process of claim 1wherein said contacting step of the improvement is carried out byplacing said printed wire board in an aqueous alkaline bath containingsaid alkali metal borate at a temperature from about 30° C. to about 60°C. and said solution comprising water and about 2 to about 50 grams ofsodium borate per liter of water having a pH from about 9.5 to about11.5.
 7. The process of claim 6 wherein said liquid carbon blackdispersion further comprises a sufficient amount of at least onealkaline hydroxide to raise the pH of said liquid dispersion in therange from about 10 to
 12. 8. The process of claim 7 wherein saidalkaline hydroxide is potassium hydroxide.
 9. The process of claim 8wherein said liquid dispersion contains less than about 10% by weightsolids constituents.
 10. The process of claim 7 wherein said carbonblack particles have an initial pH from about 2 to about
 4. 11. Theprocess of claim 7 wherein said printed wiring board is contacted with acleaner and a conditioner before step (a).
 12. The process of claim 7wherein said surfactant is a phosphate ester anionic surfactant.
 13. Theprocess of claim 7 wherein said conductive metal is copper.
 14. Theprocess of claim 7 wherein said liquid dispersing medium is water. 15.The process of claim 7 wherein said microetch in step (c) comprisessodium persulfate and sulfuric acid.